The purpose of this research project is to gain greater insight into outstanding questions regarding primary hyperparathroidism, a relatively common disorder of calcium metabolism. Despite an era of explosive growth in our knowledge of the endocrine system that helps to regulate calcium homeostatis, our appraoch to the patient with primary hyperparathyroidism is uncertain with respect to natural history, pathophysiology, objective assessment, and clinical decision making. The research project will attempt to improve our understanding in these areas with a standardized, longitudinal approach to a sufficiently large number of patients with primary hyperparathyroidism. Taking advantage of recent important advances in the measurement of circulating calcium-regulating hormones, and in the quantitative evaluation of static, dynamic, and densitometric properties of the skeleton, we plan to accomplish five specific aims: 1. greater knowledge of the natural history of primary hyperparathyroidism; 2. pathophysiological subclassification of the disease and its prognostic value; 3. comparison of complete evaluation with routing clinical assessment; 4. reversibility of the hyperparathyroid state; 5. predictive value of certain features regarding the development or prevention of complications. The ultimate goal of the research plan is to increase our current understanding of primary hyperparathyroidism in order to make our clinical approach to he patient afflicted with this disorder more secure.